Effects of man-made berms upon plant communities in New England salt marshes

Abstract

New England has an extensive history in restricting salt marsh tidal flooding to promote agricultural, industrial, and environmental endeavors. While previous research has focused on the physical and biological impacts of large-scale tidal restrictions, such as dikes and undersized culverts, the effect of smaller historic earthen barriers (average height = 0.71 m ± 0.12 SE; average length = 166 m ± 41 SE), or berms, is less understood. Here, we investigate how salt marsh plant communities respond to berms located in the interior of the marsh and oriented parallel to tidal rivers or creeks. Based on the observations from a descriptive study, the landward side of the berm consistently shows a distinct plant species assemblage from the reference areas (ANOSIM: R = 0.541, p = 0.001), which is most likely a result of landward pool development. A follow-up manipulative transplant experiment considers how the landward pools affect the governing factors (e.g., physical stress, competition, etc.) controlling the distribution and abundance of Schoenoplectus maritimus and Spartina patens in the landward and seaward zones. The experimental results show that while physical stress seems to prevent robust S. patens growth in the landward zone (ANOVA: F = 24.697; p < 0.001), herbivory seems to be the main driving factor behind the low S. maritimus cover found in the seaward zone (Mann–Whitney: U = 56, p = 0.015). The combined results from the two studies show that berm-associated pools have the potential to impact biological interactions within and across trophic levels in salt marshes.